This invention relates to apparatus for measuring the specific gravity of a liquid and, more particularly, to a method and apparatus for continuously monitoring the acid gravity in an inorganic acid-organic fluid system.
Many commercial reaction processes require the presence of an inorganic acid catalyst. For example, in petroleum refining, alkylation is widely used to convert olefinic hydrocarbons to highly branched paraffinic hydrocarbons. Because there is inevitably a certain amount of contamination of the acid catalyst in the alkylation process, it is necessary to continuously introduce make-up acid into the alkylation reactor. The quantity of make-up acid which is added to the reactor is determined on the basis of the acid strength of the acid which is separated out from the product alkylate and partly returned to the reactor. This acid strength is determined by an operator withdrawing a sample of the acid and then measuring the specific gravity of the sample which is directly related to acid strength. An optimum acid strength is normally associated with a particular process. The quantity of make-up acid introduced into the alkylation reactor is either increased or decreased to maintain such an optimum concentration in accordance with acid strength analysis.
The primary disadvantage associated with previous analyzing procedures is that the analysis is generally not continuous and regardless of the skill of the operator, the quantity of make-up acid introduced is not maintained at an optimum level. When an operator using old procedures adds make-up acid in response to a low specific gravity reading, he invariably adds too much and an amount of excess acid is present in the reactor which excess eventually becomes wasted. On the other hand, when the operator determines that the quantity of acid in the reactor has exceeded the optimum level, he reduces the amount of make-up acid and often overcompensates thus resulting in acid deficiencies in the reactor which reduces the quality of the product alkylate.